This invention relates to the preparation of a bisphenol. In one aspect, the invention relates to improving purity and yield in a process to manufacture a bisphenol employing an acidic ion exchange resin as an isomerization catalyst.
Bisphenols are used as the starting material in the manufacture of resins such as polycarbonate resins and epoxy resins. It is important that the bisphenol starting material be as pure as possible in order to avoid adverse effects on the properties of the resulting resins.
Bisphenols can be manufactured over a strong acid catalyst such as HCl or an acidic ion exchange resin catalyst by the condensation reaction of a phenol with a ketone or an aldehyde. During the preparation process, bisphenol by-products are formed, reducing the yield of the desired bisphenol. It is known that these by-products such as the o, p'- and o,o'-bisphenol isomers, can be isomerized, and related by products such as trisphenols reverted to the desired p,p'- bisphenol product by contacting a mixed product stream with an acidic ion exchange resin isomerization catalyst, thus increasing the yield of the desired bisphenol.
In certain bisphenol preparation processes, the mixed product stream containing the bisphenol in solution is passed to a crystallization zone in which the bisphenol is recovered from the solution as a solid and a portion of the remaining liquid, or "mother liquor," is passed to a second crystallization zone for further removal of bisphenol and subsequent passage through an ion exchange resin isomerization catalyst for conversion of by-products to the desired bisphenol. The isomerization product stream, minus a small purge to prevent build-up of unconvertible by-products in the recycle system, is then recycled to the second crystallization zone.
However, it has been found that acidic resin fines and strong acids leach from the isomerization catalyst into the isomerization reaction effluent. These resin fines and soluble acidity catalyze cracking of the bisphenol during subsequent purification and finishing steps resulting in lower product purity and a decrease in product yield.
In order to obtain bisphenols with higher purity, it is known to use an amine-based organic anion exchange resin to remove acidic impurities from the mother liquor. Such amine-based resins are expensive and inherently less stable than the catalyst resin, and will not be suitable for use as a catalyst resin fines filter. Use of an amine-based resin can also result in the presence of soluble amines or the reaction products of these amines with phenol in the product stream, which will decrease product quality. When the amine-based resin is used in a recycled system, the soluble amines will in turn poison the acidic ion exchange catalyst upon recycle of unconverted reactant. Such amine-based organic resins are typically regenerated by aqueous base, which is also a poison for the acidic ion exchange resin catalyst.
It is therefore an object of the present invention to provide an acid-catalyzed bisphenol preparation process employing an acidic resin catalyzed isomerization process with improved product purity and yield.